Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience.
In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, acoustic, thermal etc), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
Capacitive fingerprint sensors generally provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on the surface of the fingerprint sensor.
U.S. Pat. No. 9,383,876 discloses a fingerprint sensing system comprising: a sensor array having a plurality of sensing structures; and read-out circuitry connected to each of the sensing structures for providing sensing signals; and power supply circuitry arranged to provide to the read-out circuitry a substantially constant supply voltage being a difference between a high driving voltage potential and a low driving voltage potential. The fingerprint sensing system according to U.S. Pat. No. 9,383,876 is configured in such a way that, during operation of the fingerprint sensing system, the low driving voltage potential and the high driving voltage potential oscillate in phase in relation to a reference potential of a device comprising the fingerprint sensing system, while substantially maintaining the supply voltage.
U.S. Pat. No. 9,152,841 discloses a fingerprint sensing system including excitation signal providing circuitry coupled to the sensing structure of each sensing element for changing a potential of the sensing structure, to thereby provide a change in potential difference between the finger and the sensing structure. Based on the output from the sensing elements when this change in potential difference is provided, a representation of the fingerprint pattern of the finger can be determined.
Although fingerprint systems such as those described above can achieve excellent fingerprint image quality, there is still room for improvement, in particular for very small fingerprint sensors.